1. Field of the Invention
The present invention relates to a holder driving device which is used, for example, in an ion-implantation apparatus of a hybrid scan system, etc., and which drives mechanically a holder holding a substrate to reciprocate linearly in a vacuum vessel, and more specifically, it relates to a unit for preventing moisture in the atmosphere from flowing into its bearing unit using a differential pumping system to thereby prevent the vacuum seal performance of the bearing unit from lowering.
2. Description of the Related Art
For example, an ion-implantation apparatus of a hybrid scan system is designed to scan an ion beam reciprocatingly in a vacuum vessel in one direction, and at the same time to scan a substrate reciprocatingly mechanically in the direction substantially perpendicular to the scanning direction of the ion beam so as to perform uniform ion-implantation in the whole surface of the substrate. A holder driving device for driving a holder holding the substrate linearly and reciprocatingly in the vacuum vessel is used in such an ion-implantation apparatus.
Although it can be considered that a packing such as an O-ring or the like is used for vacuum seal in the portion where a shaft of such a holder driving device is penetrating the vacuum vessel, it is difficult to increase the driving speed of the shaft with such a packing, and it is also necessary to exchange the packing periodically because of abrasion. To this end, there is a case where differential pumping is used. In the above example, the differential pumping means a system where one or more stages of exhaust chambers are provided between the vacuum vessel and the atmosphere, and the exhaust chambers are pumped by a vacuum pump, so that the pressure in the exhaust chambers is set to values between the pressure in the vacuum vessel and the atmospheric pressure. With such differential pumping, the vacuum seal in the above-mentioned penetration portion can be attained without using any packing.
FIG. 5 shows an example of an ion-implantation apparatus of a hybrid scan system having a conventional holder driving device using such differential pumping.
This ion-implantation apparatus is configured so that in a vacuum vessel 4, an ion beam 2 travelling from the surface of the paper toward the back is made to perform scanning reciprocatingly electrically (that is, by means of an electric field or a magnetic field) in the direction X (for example, the horizontal direction, and that applies to the following description), while a holder 12 holding a substrate 6 (for example, a semiconductor wafer) is made to perform scanning reciprocatingly mechanically in the direction Y substantially perpendicular to the above-mentioned direction X (for example, in the vertical direction, and that applies to the following description) by means of a holder driving device 10 so as to radiate the ion beam 2 over the whole surface of the substrate 6 uniformly to perform ion-implantation therein.
The vacuum vessel 4 is disposed in the atmosphere, and the inside thereof is pumped up to a high vacuum, for example,about 1xc3x9710xe2x88x926 Torr to 1xc3x9710xe2x88x927 Torr, by means of a not-shown vacuum pump.
This holder driving device 10 is that which is disclosed in U.S. Pat. No. 4,726,689 and which comprises the above-mentioned holder 12, a shaft 14 supporting the holder 12 and penetrating the vacuum vessel 4, a driver (not-shown) provided outside the vacuum vessel 4 and for driving the shaft 14 linearly and reciprocatingly in the direction Y, a gas bearing unit 16 attached to the outside of the vacuum vessel 4 so as to vacuum-seal the portion where the shaft 14 penetrates the vacuum vessel 4, and a vacuum pump and a compressed gas supply means (both not shown) for this gas bearing unit 16.
The gas bearing unit 16 is constituted by a combination of a gas bearing and differential pumping. That is, this gas bearing unit 16 has plural stages (four stages in the illustrated example) of exhaust chambers 18 to 21 surrounding the shaft 14 in a housing (also called a sleeve) 17 of the unit 16. The respective exhaust chambers 18 to 21 are pumped by not-shown four vacuum pumps respectively so that they become higher in the degree of vacuum toward the vacuum vessel 4 side. Consequently, the above-mentioned differential pumping is realized.
This gas bearing unit 16 is further configured so that compressed gas 24 supplied from a not-shown compressed gas supply means is fed through an entrance 22 to a gap with its size managed accurately between a housing 17 and the shaft 14 to form a gas layer which is extremely thin (for example, about 13 xcexcm or less) in the gap so as to make the axes of the housing 17 and the shaft 14 coincident with each other to thereby make the shaft 14 held without contacting. Thus, gas bearing is realized. The pressure of the compressed gas 24 fed to the entrance 22 is usually about 6 kgf/cm2 in terms of gauge pressure. Most of the compressed gas 24 fed to the entrance 22 is discharged to the outside through an exhaust port 26 provided between the entrance 22 and the exhaust chamber 21, and the rest of the gas 24 leaks out to the atmosphere through a gap in the lower end portion of the housing 17.
In order to improve the vacuum seal performance in the gas bearing unit 16 using such differential pumping, it is generally necessary to adopt and successfully combine methods such as a method in which a gap portion is elongated in the direction along the shaft 14 between the shaft 14 and the housing 17 in order to reduce the conductance between the shaft 14 and the housing 17, a method in which a number of exhaust chambers are provided to thereby increase the number of stages of differential pumping, and a method in which vacuum pumps which are high in pumping speed are used.
The method using vacuum pumps which are high in pumping speed increases the cost. Either of the method in which a gap portion is elongated and the method in which the number of stages of differential pumping is made large causes the length of the gas bearing unit 16 to be elongated in the direction along the shaft 14 to thereby increase the lengthwise size of the holder driving device 10.
However, in such an ion-implantation apparatus of a hybrid scan system, beam line height (height from a floor surface 28 to a beam line 3) H to scan with the ion beam 2 cannot be made so high, because of the limitation of the handling height of the substrate 6, the maintenance height of an ion source for emitting the ion beam 2, the total height of the ion-implantation apparatus, and so on. Therefore, the lengthwise size of the holder driving device 10 also has a limit.
Therefore, there is a limit in increasing the number of stages of differential pumping in the gas bearing unit 16 or in elongating the length of the gap portion in the direction along the shaft 14.
In the gas bearing unit 16 configured under such a limitation, it was found that a good vacuum seal performance is not maintained and resulted in a bad vacuum in the vacuum vessel 4 if the driving rate is increased when the shaft 14 was driven reciprocatingly and linearly as mentioned above, while the degree of vacuum in the vacuum vessel 4 was not influenced when the shaft 14 was not driven.
For example, the maximum rate of scanning the holder 12 mechanically in the direction Y usually reaches about 250 to 300 mm/sec. Even in that case, it is necessary to suppress the deterioration of the degree of vacuum in the vacuum vessel 4 to be not larger than about 10% of the ultimate degree of vacuum (for example, about 1 to 5xc3x97xe2x88x927 Torr), but this is difficult to realize for the above-mentioned gas bearing unit 16. In the gas bearing unit 16, the scanning rate of the holder 12 is limited to about 50 mm/sec in order to suppress the deterioration of the degree of vacuum to be within the above-mentioned range.
In order to find out the reasons for the deterioration of the degree of vacuum, the composition of gas in the vacuum vessel 4 when the degree of vacuum deteriorated was investigated by using a quadrupole mass spectrometer and as a result it was found that moisture entered the vacuum vessel 4 together with the shaft 14 when the shaft 14 entered the vacuum vessel 4. It is considered that the deterioration of the degree of vacuum is caused by the fact that moisture contained in the atmosphere enters the first stage (atmosphere-side) exhaust chamber 21 and changes therein into waterdrops (or ice pieces) because of the decrease of temperature caused by adiabatic expansion of the moisture, and then the waterdrops (or ice pieces) enter the vacuum vessel 4 through the other exhaust chambers 20 to 18 successively. If the moisture changes into waterdrops or ice pieces, it is difficult to pump those waterdrops or ice pieces in the next stages of exhaust chambers 20 to 18.
Although there is a thought to use also a packing such as an O-ring or the like in the gas bearing unit 16, the packing slides linearly relative to the shaft 14, so that it is difficult for the packing to prevent moisture from passing from between the shaft 14 and the packing.
It is therefore a main object of the present invention to provide a holder driving device in which it is possible to prevent moisture in the atmosphere from flowing into a first stage exhaust chamber of a bearing unit using differential pumping, so that it is possible to prevent the vacuum seal performance in the bearing unit from deteriorating when a shaft is driven reciprocatingly and linearly at a high speed.
The foregoing object of the invention is achieved by providing a holder driving device having: a holder holding a substrate, the holder provided in a vacuum vessel disposed in the atmosphere; a shaft supporting the holder and penetrating the vacuum vessel; a driver provided outside the vacuum vessel and driving the shaft reciprocatingly and linearly in an axial direction of the shaft; a bearing unit attached on the outside of the vacuum vessel so that the shaft passes through the bearing unit, and the bearing unit having at least one of mechanical direct-acting bearings for supporting the shaft and at least one of exhaust chambers surrounding the shaft; a vacuum pump for vacuum-pumping the exhaust chambers of the bearing unit; and a gas replacement mechanism for supplying dry gas in the neighborhood of an atmosphere-side entrance portion of a gap between an end portion of the bearing unit of a side opposite to the vacuum vessel and the shaft passing the end to portion, so as to replace the atmosphere in the vicinity of the entrance portion by the dry gas, the dry gas being lower in humidity than at least the surrounding atmosphere and being a positive pressure.
Preferably, the gas replacement mechanism is constituted by a purge chamber provided so as to partition between the end portion of the bearing unit on the side opposite to the vacuum vessel and the atmosphere side and so as to surround the shaft passing the end portion, and a gas supply means for supplying the purge chamber with dry gas the humidity of which is lower than that of at least the surrounding atmosphere and the pressure of which is positive.
Alternatively, it is preferable to constitute the gas replacement mechanism by an isolation vessel, and a gas supply means for supplying the isolation vessel with dry gas which is lower in humidity than that of at least the surrounding atmosphere and which has positive pressure. The isolation vessel surrounds, with separation from the atmosphere, the end portion of the bearing unit on the side opposite to the vacuum vessel, the shaft projecting outside from the end portion, and the whole of the periphery of the driving device for driving the shaft.
According to the above-mentioned configuration, at least the atmosphere near the entrance portion of the gap in the atmosphere-side end portion of the bearing unit using differential pumping can be replaced by the dry gas by means of the gas replacement mechanism, so that it is possible to prevent the atmosphere from flowing into the first stage exhaust chamber of the bearing unit to thereby prevent moisture in the atmosphere from flowing into the first stage exhaust chamber. Accordingly, it is possible to prevent the vacuum seal performance in the bearing unit from being deteriorated even when the shaft is driven so as to reciprocate linearly at a high speed.